The long term objective of this project is to elucidate mechanisms of determination in animal embryonic development, using the nematode Caenorhabditis elegans as an experimental system. In the C elegans embryo, determination of cell fates is a hierarchical process: the primary determinants are probably localized, maternally derived macromolecules, whose effects are secondarily modified by establishment of sexual identity and by cell interactions controlling expression of embryonic genes that further dictate cell-type determination. This project will continue to investigate the mechanisms of sex determination and the roles of embryonically expressed genes in patterning and cell fate decisions. In the first area, recent work from our laboratory has shown that her-1, the initial major switch gene in the regulatory pathway that controls sex determination, encodes a small secreted polypeptide that exerts its masculinizing effects via transmembrane signalling. To further elucidate her-1 function, the regulation of its male-specific transcription will be investigated, the active form of the her-1 product will be isolated and its primary structure determined, and experiments will be undertaken to identify its receptor by binding assays with embryonic cells in culture. These studies could have eventual health implications for control of human parasitic nematode infections. In a related project, a small region of the X chromosome that appears to be a component of the primary signal for sex determination will be molecularly characterized. In the area of cell-type determination, ongoing large-scale screens using trimethyl-psoralen as a mutagen will be continued, with the goal of saturation for non-maternal-effect mutations that result in specific effects on embryonic patterning. In the course of these screens, several loci (nob genes) have been identified that mutate to cause similar severe posterior patterning defects, and one of these has been shown to be a previously identified homeobox gene (pal-1), homologous to the caudal gene in Drosophila. The remaining nob genes will be cloned for molecular characterization, and their interactions with each other and with other known embryonically expressed and maternal-effect genes will be investigated by genetic as well as molecular methods, in an effort to understand the control of posterior patterning in the embryo. In light of accumulating evidence that all animal embryos employ similar determination mechanisms, these studies are likely to contribute eventually to understanding of human hereditary birth defects and other developmental disorders.